In many industrial environments, products are processed and/or stored in tanks or bins. Examples include grain silos, tanks used in the batch processing of foods, beverages, or pharmaceuticals. In these industries, one must be able to reliably determine the amount of contents stored within a tank at any given time. This determination may be made visually or through the use of a level sensing gauge. In many instances, the tank prevents any sort of visual determination of the present level of the contents. For example, many silos are composed of steel and/or concrete or other nontransparent materials and, therefore, any visual inspection of the content level of such tanks would involve manually opening an aperture, which may pose hazards to personnel, and raise the potential of contamination of the contents. Furthermore, visual inspection of content levels lacks accuracy and is time consuming. In order to avoid these problems, level sensing gauges are used to measure content levels.
There are several types of level sensing gauges. Examples include those that use mechanical systems and electrical systems, with the electrical versions including radar transmitters or ultrasonic systems, and others. Radar gauges in particular monitor content levels by transmitting microwave pulses from an antenna toward the surface of the tank contents. These pulses are reflected from the contents and back to the antenna. The antenna cone or rod-shaped. Other radar gauges use a continuous wave rather than pulses with a similar antenna.
Radar sensors are suitable for measuring levels of liquids, solids, powders, granules, dust, corrosive steam and vapors, regardless of the media characteristics, environment, low and high pressures or temperatures. Radar signals are unaffected by noise, by extreme air turbulence (such as during pneumatic filling), by fluctuations in dielectric constant, density, or conductivity. Even liquids having highly agitated surfaces or gas bubbles are usually reliably measured. Gas layering such as that produced by solvents or gases has virtually no adverse effect.
Although radar signals are generally useful for measurement of multiple materials, radar measurement gauges are sensitive devices and prone to malfunction when exposed to dust or condensation, particularly in the transmitting and receiving antenna. Particles of dust within the antenna may create false echoes preventing reliable measurement of level, and drops of fluid in the radar transmission path may have a similar effect.
U.S. patent application Ser. No. 09/178,836, filed Oct. 6, 1998, now U.S. Pat. No. 7,134,315 issued Nov. 14, 2006, assigned to the same assignee as this application and incorporated herein in its entirety, describes a mounting for a radar level sensing gauge sold by the assignee, that is particularly intended for sanitary environments such as food processing. For this application, a hole is first cut into the body of a storage tank, generally the roof. Next, the level sensing gauge is then inserted through the hole into the tank with its probe antenna extending into the tank inside of a probe cover. The probe cover includes a sealing flange that seals to the housing of the tank to prevent contamination of the contents of the tank through the hole. The probe is positioned beneath the surface of the tank when the level sensing gauge is attached to the tank, thereby reducing the disruption of radar emissions by the tank itself.
While the above-referenced patent application provides an effective solution to prevent contamination of the contents of a tank, it does not provide a solution for level sensing gauges that utilize a cone-shaped antenna, nor does it provide a seal usable on such a gauge to prevent entrapment of dust or moisture from outside of the tank which could cause false echoes and malfunction of the level sensing gauge. Dust and moisture from such sources is particularly problematic on grain silos which operate in dusty, outdoor environments that are subject to extreme temperature and humidity variations.
In view of the above background, there is need for a level sensing gauge which is usable with a storage tank in a dusty, temperate and moist environment, to prevent the ingress of dust or moisture into the antenna structure and malfunctions resulting therefrom. Further, it is desirable that as the level sensing gauge is mounted to a storage tank the antenna is positioned within the tank to reduce interference between the radar signal and the body of the tank.